Recent progress in technology has provided convenient ways to use digital data without loss of quality. Many kinds of content are available as digital data, such as digital pictures or music, and this data can be manipulated in various ways, such as creating, storing, copying, editing, and exchanging. At the same time, protecting the content from undesired copying or other use has become more difficult for the owner of the underlying content.
One type of approach in controlling distribution of digital data is called revocation schemes or broadcast encryption schemes. A sender sends encrypted information or content to a group of receivers over a broadcast channel. One or more of the receivers are not authorized to decrypt the information. The unauthorized receivers are also called revoked receivers. The revoked receivers do not have a decryption key matching the encryption of the broadcast encrypted information. All of the receivers receive the information, but some receivers will be able to decrypt the content while unauthorized or revoked receivers will not. Examples of uses of revocation schemes include pay television systems and copy-protected media.